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J 4 United States Pam Ofiice Re. 24,062 Reissuecl Sept. 30,. 1955 PROCESS FOR TREATING POLYETIIYLENE AND COATED PRODUCT Pllll V. [ml] P I" N. Y J to M p mihiflommssotnenware,

No Drawing. Original No. 2,668,134, dated February 2,

1954, Serlal No. 47,147, August 31, 1948. guru for reimue August 19, 1954, Serial No. 451,

11 Claims. (Cl. 154-439) Matter enclosed in heavy brackets 1 h the liw specllicatlon; matter printed in Italics indicates the additions V This invention relates to the preparation of polyethylene Q articles for cementing or printing, and more particularly tinto the oxidation of the normally hydrophobic surfaces of the articles so that they are hypophilic and free of static charge.

Heretofore, difliculty has been experienced in cementing polyethylene products and in printing legends thereon, the normally hydrophobic surface of polyethylene being unreceptive to most glues, adhesives and printing inks and dyes.

in accordance with the present invention, polyethylene surfaces readily may be made hydrophilic and adaptable to cementing and printing by oxidation with a strong sulphuric acid-dichromate solution which may be obtained by saturating concentrated sulphuric acid with technical sodium dichromate. The surface is prepared simply by passing it through a bath of the oxidizing solution and then washing in water and drying.

In the case of polyethylene sheet and similar products which are formed by continuous processes, as for example, extrusion, the surface treatment of the present invention may be integrated in the continuous production so as continuously to produce a1product having a hydrophilic surface.

The degree of treatment or modification of the normally hydrophobic surface can be regulated by varying the temperature and strength of the acid-dichromate solution and the exposure time of the surface to the'solution.

' With strong solution at room temperature (70 F.) the treatment is practically instantaneousin rendering the surface hydrophilic.

, The subsequent washing of the treated surface immediately and completely removes any sulphuric acid products which may have adhered to the polyethylene 2 way it is possible to form letters and other drapes on polyethylene by subjecting such treated areas to the action of dyes or aqueous inks. A treated surface can be printed on quite successfully with nitro cellulose lacquer. If the printed surface is submerged in water for an hour, the printing can be scratched off, but will not be damaged by rubbing.

The treatment does not inpair the optical properties 7 of the sheet and, in fact, has been found to improve considerably the transparency or clarity of polyethylene.

Metals may be easily deposited on such treated surfaces by allowing the surface to come into contact with an aqueous solution of a readily-reducible metal salt and reducing the metallic salt. Thin films of copper and silvermay be deposited in this manner. Metals may also be applied to the treated surface by the process of low vacuum volatilisation. Better adhesion is obtained when the metals are applied to treated, as compared with untreated, polyethylene surfaces.

It will be-understood that the polyethylene treatment of the present invention maybe used wherever the hydrophilic property is required or helpful. For example, the interior of polyethylene bottles treated with the aciddichromate has been, successfully given an adherent chain as a result of the sulphuric acid-dichromate treatment. Water is found to spread evenly over a polyethylene surface treated in this way instead of gathering in drops as in the case of an untreated surface. Subsequent exposure of a treated surface to water does not wash away or destroy the hydrophilic character of the treated surface. The treated surface will take up basic dyes from an aqueous solution, while adhesives and cements based on polyvinyl alcohol, polyvinyl acetate, nitro cellulose, gelatin, Hycar rubber and similar polar compounds adhere readily to such a surface. Hycar rubber is the name given by B. F. Goodrich Company to a synthetic rubber compound that is a butadiene copolymer of undisclosed composition. Solvents which migrate readily through polyethylene as, for example, methyl ethyl ketone, can be used as solvents for the cement.

It will be obvious that the whole of the surface of the polyethylene article need not be treated with the aciddichrornate but that areasofvarious disposer form canbesubiectedtotheactionofthesolution. Inthis oil resistant film of polyvinyl alcohol. It was found that the adherence of the him was great enough to tear the surface of the polyethylene. I

It also has been found that electrostatic charges do not accumulate on polyethylene surfaces treated according to the process of the present invention and polyethylene thus treated can advantageously be used in-the manufacture of covers, for instruments, dials, gauges, indicators and the like, especially those in which accumulation of electrostatic charges on the cover has a disturbing influence on the instrument itself.

I claim:

l. The process of treating polyethylene products which comprises exposing the product to a saturated solution of sodium dichromate in concentrated sulphuric acid to render the surface of the polyethylene hydrophilic, receptive to printing ink, and free of static electricity.

2. The process of treating polyethylene products which.

comprises exposing the product to a bath of sulphuric acid and sodium dichromate, washing the product in water and drying. 1

3. The process recited in claim 2 wherein the product is exposed only instantaneously to the acid-dichromate.

4. The process recited in claim 2 and including exposing the product to a dye.

5. The process of treating polyethylene products which comprises exposing the product to a bath of sulphuric acid and sodium dichromate, washing, drying, applying a nitro cellulose lacquer.

6. The process of bonding a polyethylene surface to another surface which comprises exposing the polyethylene surface to a bath of sulphuric acid and sodium dichromate, washing, and bonding the surfaces together with a polar cement.

7. Solid polyethylene having a surface which is bydrophilic, receptive to printing inks, and free of static charge, said surface comprising essentially oxdired polyme and which surface has an adherent film of an oil I 2,413,789 Seheiderbauer Jan. 7, 1947 resistant polar compound. v 2,418,018 Erusberger et a1 Mar. 25, 1947 II. A polyethylene article, a surface of which com- 2,431,042 I ngersoll Nov. 18, 1947 1" I prises polar compound receptive; oxidized polyethylene 2,431,956 Moody Dec. 2, 1947 1 and which surface has an adherent film of an oil re 5 2,446,536 Hardy Aug. 10, 1948 i sis-tan: polar compound. 2,584,722 London Feb. 5, 1952 References Cited in the file of, this patent OTHER REFERENCES 7 orthe original patent British Plastics, Properties and Uses of Polythene,"

- 1 Richards, April 1945, pp. 146 to 151.

v UNITED STATES PATENTS Modern Inorganic Chemistry," Mellor, 1917, p. 465. g v 2,125-374 Herman al B- 2, 1938 Industrial and Engineering Chem, Polythene, June 3 2,153,553 Fawcet et al Apr. 11, 1939 7 1945 pp. 52541 1315-196. Schlack Sepl- 1940 :British Plastics, Properties and Uses of Polythene, 2,341,885 Feb-15,1944 15 Midwinter, May 1945, pp. 208-228. 7

2,400,720 S tandinger et a1; May 21, 1946 

